How to resolve the algorithm Base64 decode data step by step in the C++ programming language

Published on 7 June 2024 03:52 AM
#cpp

How to resolve the algorithm Base64 decode data step by step in the C++ programming language

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Problem Statement

See Base64 encode data. Now write a program that takes the output of the Base64 encode data task as input and regenerate the original file. When working on the VBA implementation I found several 'solutions' on the net, including one from the software maker himself, that showed output with incorrect padding. Obviously with incorrect padding in the output you can not decode correctly to the original file again.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Base64 decode data step by step in the C++ programming language

The code snippet you provided is an implementation of the Base64 encoding and decoding algorithm in C++. Base64 is a binary-to-text encoding scheme that represents binary data in an ASCII string format.

The encode function takes a vector of bytes as input and returns a vector of bytes that represents the Base64 encoding of the input. The function iterates over the input bytes, three bytes at a time, and converts each group of three bytes into four Base64 characters. The function uses the BASE64 constant, which is a string of the 64 characters that are used in the Base64 encoding scheme.

The decode function takes a vector of bytes as input and returns a vector of bytes that represents the decoded binary data. The function iterates over the input bytes, four bytes at a time, and converts each group of four bytes into three bytes of binary data. The function uses the findIndex function to convert each Base64 character into its corresponding index in the BASE64 constant.

The main function takes a string as input and encodes and decodes the string using the encode and decode functions. The function then prints the decoded string to the console.

Here is a more detailed explanation of the code:

  • The encode function iterates over the input bytes, three bytes at a time. For each group of three bytes, the function does the following:

    • Converts the first byte into a six-bit number.
    • Converts the second byte into a four-bit number.
    • Converts the third byte into a two-bit number.
    • Concatenates the three numbers to form a 12-bit number.
    • Divides the 12-bit number into four six-bit numbers.
    • Converts each six-bit number into a Base64 character.
    • Adds the four Base64 characters to the output vector.

  • The decode function iterates over the input bytes, four bytes at a time. For each group of four bytes, the function does the following:

    • Converts the first byte into a six-bit number.
    • Converts the second byte into a four-bit number.
    • Converts the third byte into a two-bit number.
    • Converts the fourth byte into a two-bit number.
    • Concatenates the four numbers to form a 12-bit number.
    • Divides the 12-bit number into three eight-bit numbers.
    • Converts each eight-bit number into a byte.
    • Adds the three bytes to the output vector.

  • The main function takes a string as input and encodes and decodes the string using the encode and decode functions. The function then prints the decoded string to the console.

Source code in the cpp programming language

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

typedef unsigned char ubyte;
const auto BASE64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

std::vector<ubyte> encode(const std::vector<ubyte>& source) {
    auto it = source.cbegin();
    auto end = source.cend();

    std::vector<ubyte> sink;
    while (it != end) {
        auto b1 = *it++;
        int acc;

        sink.push_back(BASE64[b1 >> 2]);            // first output (first six bits from b1)

        acc = (b1 & 0x3) << 4;                      // last two bits from b1
        if (it != end) {
            auto b2 = *it++;
            acc |= (b2 >> 4);                       // first four bits from b2

            sink.push_back(BASE64[acc]);            // second output

            acc = (b2 & 0xF) << 2;                  // last four bits from b2
            if (it != end) {
                auto b3 = *it++;
                acc |= (b3 >> 6);                   // first two bits from b3

                sink.push_back(BASE64[acc]);        // third output
                sink.push_back(BASE64[b3 & 0x3F]);  // fouth output (final six bits from b3)
            } else {
                sink.push_back(BASE64[acc]);        // third output
                sink.push_back('=');                // fourth output (1 byte padding)
            }
        } else {
            sink.push_back(BASE64[acc]);            // second output
            sink.push_back('=');                    // third output (first padding byte)
            sink.push_back('=');                    // fourth output (second padding byte)
        }
    }
    return sink;
}

int findIndex(ubyte val) {
    if ('A' <= val && val <= 'Z') {
        return val - 'A';
    }
    if ('a' <= val && val <= 'z') {
        return val - 'a' + 26;
    }
    if ('0' <= val && val <= '9') {
        return val - '0' + 52;
    }
    if ('+' == val) {
        return 62;
    }
    if ('/' == val) {
        return 63;
    }
    return -1;
}

std::vector<ubyte> decode(const std::vector<ubyte>& source) {
    if (source.size() % 4 != 0) {
        throw new std::runtime_error("Error in size to the decode method");
    }

    auto it = source.cbegin();
    auto end = source.cend();

    std::vector<ubyte> sink;
    while (it != end) {
        auto b1 = *it++;
        auto b2 = *it++;
        auto b3 = *it++; // might be first padding byte
        auto b4 = *it++; // might be first or second padding byte

        auto i1 = findIndex(b1);
        auto i2 = findIndex(b2);
        auto acc = i1 << 2;     // six bits came from the first byte
        acc |= i2 >> 4;         // two bits came from the first byte

        sink.push_back(acc);    // output the first byte

        if (b3 != '=') {
            auto i3 = findIndex(b3);

            acc = (i2 & 0xF) << 4;  // four bits came from the second byte
            acc |= i3 >> 2;         // four bits came from the second byte

            sink.push_back(acc);    // output the second byte

            if (b4 != '=') {
                auto i4 = findIndex(b4);

                acc = (i3 & 0x3) << 6;  // two bits came from the third byte
                acc |= i4;              // six bits came from the third byte

                sink.push_back(acc);    // output the third byte
            }
        }
    }
    return sink;
}

int main() {
    using namespace std;

    string data = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo";
    vector<ubyte> datav{ begin(data), end(data) };
    cout << data << "\n\n" << decode(datav).data() << endl;

    return 0;
}

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